Studies of vapor grown carbon fiber started in the latter 1980s and it has been found that when a gas such as hydrocarbon is thermally decomposed in vapor phase in the presence of a metal catalyst, a carbon fiber having a diameter of 1,000 nm or less and a length of about several tens of μm is obtained.
For example, there is disclosed a method where an organic compound such as benzene is used as the raw material and an organic transition metal compound such as ferrocene is introduced as the catalyst precursor together with a carrier gas into a high-temperature reaction furnace to produce a carbon fiber on a substrate (Japanese Unexamined Patent Publication (Kokai) No. 60-27700), to produce a vapor grown carbon fiber in the suspended state (Japanese Unexamined Patent Publication (Kokai) No. 60-54998) or to grow a carbon fiber on a reaction furnace wall (Japanese Patent No. 2778434).
According to this production method, a relatively thin carbon fiber excellent in electric or thermal conductivity and having a large aspect ratio is obtained and carbon fibers having a fiber outer diameter of about 10 to 200 nm and an aspect ratio of about 10 to 500 are already being mass-produced.
Also, as a carbon fiber thinner than such a vapor grown carbon fiber, Iijima et al., have discovered a carbon nanotube from a soot obtained by evaporating a carbon electrode by arc discharge in a helium gas. This carbon nanotube is a linear fiber having a diameter of 1 to 30 nm and a large aspect ratio and this is a fine carbon fiber where similarly to the vapor grown carbon fiber, hexagonal carbon layers are multiply stacked around the fiber axis like a tree-growth-ring and closed at the tip and the inside thereof is hollow.
Other than this linear carbon fiber, a crimped carbon fiber is also known. For example, in Japanese Unexamined Patent Publication (Kokai) No. 61-225319, a carbon fiber having a solid and tree-growth-ring structure and having a percentage crimp of 0.5 to 50%, a fiber outer diameter of 0.05 to 4 μm and an aspect ratio of 100 or more is provided. It is disclosed that this crimped carbon fiber easily forms a network among fibers in a matrix as compared with the linear carbon fiber and by virtue of crimp, fibers are entangled with each other when filled as a filler in metal, resin, ceramic or the like, so that the contacting ratio increases and in turn the electric conductivity is enhanced. However, this carbon fiber has an aspect ratio as large as 100 or more, and therefore has a problem in dispersibility in metal, resin, ceramic or the like. Journal of Catalysis, 30, 86-95 (1973) similarly discloses a crimped fiber, but the synthesis thereof is performed under very specific and non-industrial conditions, that is, in an electron microscope and also, the structure and properties of the carbon fiber obtained are not disclosed.
A carbon fiber having a large aspect ratio has a problem in dispersibility at kneading as a filler material with a matrix component such as metal, resin or ceramic, and is disadvantageous in that uniform dispersion in a matrix is difficult to obtain, an excessive amount of filler is necessary for obtaining desired properties, a special dispersing machine is required for the dispersion and the profitability is low.
In order to enhance the dispersibility of a carbon fiber having a large aspect ratio, a technique of mechanically cutting the carbon fiber by grinding or the like or shortening the fiber length by a chemical treatment is known. However, such a technique is deficient not only in that an extra step is necessary and the profitability is low, but also in that linear short fibers can hardly form a network among fibers and the properties such as electric conductivity are not satisfactorily brought out.
In addition, these carbon fibers are a fiber having a structure in which hexagonal carbon layers are stacked and wound like a tree-growth-ring and a very thin hollow is present in the inside. In the carbon fiber having such a tree-growth-ring structure, the fiber surface is inactive, and therefore the carbon fiber is neither used as a filler nor as an adsorbent for occluding hydrogen, methane or various gases or a catalyst support.
With an attempt to alter the structure of these carbon fibers, a herringbone-type carbon fiber or a hollow-free carbon fiber (plate-type fiber) where carbons are vertically stacked on the fiber axis is disclosed (Langmuir., 11, 3862-3866 (1995)). These carbon fibers have a very active surface. Also, Japanese Unexamined Patent Publication (Kokai) No. 2003-73930 discloses a multilayer carbon fiber having a multilayer structure containing a hollow structure in the inside, where the carbon structure of the inner part contains a herringbone structure or a shape that carbons are vertically stacked on the fiber axis, and the carbon structure of the outer layer part contains a tree-growth-ring structure. These fibers are, similarly to conventional vapor grown carbon fibers and the like, a linear fiber having a large aspect ratio and because of their poor dispersibility on use as a filler material, these are not used in practice.
As the carbon compound analogous to the vapor grown carbon fiber, a single-walled carbon nanohorn is known and application thereof to an adsorbent for occluding hydrogen, methane or various gases or a catalyst support is disclosed (Japanese Unexamined Patent Publication (Kokai) No. 2002-159851). In such a single-walled carbon nanohorn, the diameter of the tubular part is from 2 to 3 nm, the fiber length is 30 nm and the specific surface area is about 300 m2/g. Supposing a cylindrical body, the specific surface area calculated from the fiber outer diameter is about 1,000 m2/g and this is a very fine carbon compound, but a carbon nanotube having a fiber outer diameter of several nm is estimated to have a specific surface area equal thereto and when the fiber outer diameter is taken into consideration, this carbon compound cannot be said to have a peculiar specific surface area. Accordingly, when intended to obtain a necessary specific surface area on use, for example, as a material for occluding hydrogen, methane or the like or as a catalyst support, a thin fiber is used. The cost of the thin fiber is generally high because of its low productivity, and use of such a material is unprofitable.
Similarly, a technique of activating the produced carbon fiber by subjecting it to a post treatment such as surface treatment and thereby obtaining a carbon fiber having a high specific surface area is known, but a step for surface treatment or the like is necessary and this is not profitable.
As described above, in using a carbon compound such as vapor grown carbon fiber and carbon nanotube as a filler material, there has been heretofore not present an inexpensive carbon fiber having enhanced dispersibility to express excellent properties such as high electric conductivity and high thermal conductivity and at the same time, having a very large specific surface area to be suitably used as a gas adsorption medium or a catalyst support.
Under these circumstances, an object of the present invention is to provide an inexpensive crimped carbon fiber useful as a filler material for enhancing the electric conductivity, thermal conductivity and strength, as an adsorbent for occluding hydrogen, methane or various gases or as a catalyst support.